There is an extremely great need for seamless pipes within the diameter range of 7 inches (177.8 mm) up to 26 inches (660 mm) with a ratio of the diameter to the wall thickness being within the range of 15:1 to 50:1. Such seamless pipes are predominately used, for example, in oil field applications, e.g. as drill pipes, delivery pipes or liner pipes. In this connection, the manufacture of seamless pipes of high quality, i.e., with narrow tolerances of wall thickness and diameter as well as a good surface, is relatively difficult and requires a corresponding expense for equipment. This is true in particular of thin-walled pipes having a ratio of diameter to wall thickness of more than 25:1. The methods of rolling heretofore used for the manufacture of such seamless pipes are on the one hand the piercing-rolling method and on the other the pilger rolling method. With respect to the general design of the two methods of rolling, reference is had to the Stahlrohr-Handbuch, 10th Edition, 1986, particularly pages 128 and 133.
The disadvantage of the piercing-rolling method is that in order to equalize the beads in wall-thickness coming from the lengthwise rolling and to obtain acceptable roundness, two parallel travelling smoothing rolls (reelers) and sizing or reducing rolls for reasons of output must be arranged behind the piercing rolling mill. Ordinary reelers are developed in the entrance part either as a barrel or divergent cone with an angle of reduction of up to about 2.degree. (Hutnicke listy 38 (1983) No. 11, Pages 779-782).
In the case of the last mentioned rolling step, the pre-pipe has been slightly widened by the reeling to be rolled to the desired final dimension with a corresponding reduction of its diameter. Since in such a sizing mill the decrease per stand amounts to about 2 to 4 percent, a large number of stands are required to have an ordinary standardization of parent pipes in order to achieve the corresponding decreases in diameter. Many stands, however, mean high investment expenses and a corresponding stocking of rolling stands.
In the pilger rolling method in which, for reasons of expense, standardization of the parent pipes is generally dispensed with, the last reshaping step, usually, is only a calibration ordinarily with 3 stands, but the quality of the surface, and particularly the tolerances in the wall thickness of the pilger-produced pipe, in most cases do not satisfy the increased demands.